As data communication systems such as a plurality of individual computers interconnected by a local area network (LAN) become prevalent in the business place, and increase in speed of data transmission and sophistication, the nature of the cable employed in the communication network becomes more and more critical. Many such networks employ unshielded twisted pair (UTP) cable including four pairs of conductors, terminated at each end in standard eight pin modular jack (commonly referred to as RJ45 connectors). Successful operation of high-speed data communication networks employing such UTP cable is dependent upon the quality of the cable and the manner in which it is installed. Standards are being developed to classify cable based upon the near-end crosstalk (NEXT) and attenuation characteristics of the cable. Such parameters, in conjunction with the length of cable, tend to dictate the steady state signal to noise ratios attainable by the system, and the ability of the cable to accommodate the various forms of high-speed data transmission. Near-end crosstalk performance of the cable is a measure of how much signal is coupled from one pair of transmit conductors to receive conductors in the same sheath, when both the transmitter and receiver are located at the same (near) end.
Data Communication Systems employing local area networks typically are full duplex systems; transmission and reception by a unit in the system will occur simultaneously, transmission being provided as a balanced differential signal on a first signal pair, (the transmit pair) and receiving a balanced differential signal on a second cable pair (the receive pair). Near-end crosstalk from the transmit pair to the receive pair, is perceived by the unit as noise, and denigrates system performance. Attenuation is a measure of the decrease in amplitude of signals as they propagate through a length of cable.
The near-end crosstalk performance of a cable is typically measured by injecting a reference transmit signal (test signal) on the near-end of a designated set of conductors, and detecting the amount of test signal present at the near-end cable on the other sets of conductors. This test is typically performed with the cables under test terminated in their characteristic impedance. The NEXT parameter is frequency dependent, and the test is conducted over a range of frequencies, for example, from 200 Khz to 100 Mhz.
Attenuation is measured by injecting a reference signal into a conductor pair, and measuring the decrease in amplitude in the signal received at a second point in the cable. In more sophisticated systems, the signal is injected into a twisted pair at the far end of the cable, and the attenuation measurement made at the cable near-end. Other less sophisticated systems, tend to employ what is referred to as a "loop back" method wherein the individual conductors of two twisted pairs are interconnected at the far-end, and the reference signal is injected at the near-end of one pair, and received at the near-end of the other pair. A measurement is thus made of the cumulative attenuation of the two pairs.
In many instances, it is necessary to certify whether new or existing wiring meets manufacturer specifications, or determine whether existing wiring can accommodate higher speed operation, or different communication protocol, e.g., determine if an existing UTP cabling employed in a 4 megabyte token ring system can support a 16 megabyte token ring, 10BASE-T or ARCNET network application. NEXT and attenuation measurements are required for this cable certification process.
In general, devices for testing UTP cable are known. For example, the commercially available Microtest MT-340 scanner automatically effects a series of tests on cable to certify whether or not new or existing cable will support various network applications. Such tests include, among other things, measurements of near-end crosstalk, cable length, signal attenuation and DC resistance. The MT-340 scanner engages the connector at the near end of the cable to be tested and a remote unit is connected to the far end of the cable. The remote unit, responsive to control signals provided by the MT-340 scanner through the cable, provides the appropriate termination of particular wire pair under test, or in the case of e.g., attenuation measurements, injects the appropriate signal at the far end of the cable. Measurement of near-end crosstalk between respective conductor pairs in the cable, in the MT-340 scanner system is affected as follows: the switching matrices, formed of relays in the MT-340 scanner employed to selectively couple the signal generating and receiving circuitry to respective sets of conductor pairs; commands are generated to the remote unit (which includes a switching matrix similar to that in scanner) to terminate the selected pairs in their characteristic resistance; a test signal of predetermined frequency is injected into one of the conductor pairs, and the coupled signal on the other pair are detected. The frequency of the test is incremented through a range of frequencies from 200 Khz to 20 Mhz to measure NEXT across the relevant frequency range. The crosstalk between the various pairs of the cable are measured in sequence; control signals are generated to the switching matrices to select the relevant sets of pairs.
It has now been determined, however, that a large component of the measured near-end crosstalk is due to the RJ45 connector. The NEXT performance criteria for cables used in high speed data communication networks is now exceeding the crosstalk performance of the connector system. This is particularly true with respect to cables meeting high level standards (e.g., UL LEVEL 5 cable); the crosstalk effected in the connector is sufficiently greater than the cable near-end crosstalk so as to mask the cable crosstalk, and make it unmeasurable by conventional test equipment. Examples of high performance cable specifications for NEXT are -56 dB at 16 Mhz. The typical NEXT performance of a mated RJ45 connector on certain pairs is -43 dB. If such connector crosstalk is not canceled, the test equipment will not be able to certify that the cable meets the required -56 dB NEXT specifications. The crosstalk from the connector overwhelms the crosstalk on the cable media and severely limits the measurement capability of the test equipment to the crosstalk performance of the connector. Indeed, when testing through connectors, accurate NEXT characteristics of the high performance cable can not be obtained by conventional test apparatus due to the limitations of the connector system. This is problematical; the objective of cable certification test equipment is to determine that the cable media per se; meets predefined characteristics (e.g., UL cable grades). The UL cable grading specifications relate to the cable media per se; connector effects are not provided or accounted for in the specifications. Therefore, it is highly desirable to implement some methodology in cable certification test equipment which cancel or eliminates connector crosstalk. This would then allow certification of high performance cable in a typical installation environment, i.e., terminated in a standard RJ45 modular jack.